The interest in environmental problems has been internationally elevated in recent years, and a cogeneration system employing a fuel cell or a micro gas turbine has been developed and widespread as a link thereof. High-temperature gas flows in a heat exchanger constituting this cogeneration system, and the temperature of this gas tends to increase in order to improve heat generation efficiency. In general, a material prepared by employing stainless steel for a substrate while employing nickel solder (JIS BNi-1 to 7) for a brazing filler metal is known as a heat exchanger material capable of withstanding a severe service condition under such a high temperature. Nickel solder, which is a material excellent in oxidation resistance and corrosion resistance but hard to plasticize, is generally manufactured in a powder state by a liquid quenching method. Thus, nickel solder has been inconveniently high-priced. Further, a debindering step is necessary after brazing since a pasty material prepared by mixing a binder into powdery nickel solder is applied to stainless steel forming the substrate in a manufacturing step for the heat exchanger, and there has been such inconvenience that the manufacturing step is complicated.
In the meantime, there is generally proposed a technique of manufacturing a brazing filler metal consisting of a laminated structure of an Ni-based metal layer and an Ni-based metal layer consisting of an Mn—Ni—Cu alloy by employing rolling/bonding without employing the aforementioned liquid quenching method and brazing/bonding stainless steel with this brazing filler metal. Such a technique is disclosed in International Patent Laying-Open No. WO00/18537, for example. According to this International Patent Laying-Open No. WO00/18537, the Mn—Ni—Cu alloy is made to contain Cr, Ti etc. to be not more than 5 mass % in total, thereby improving oxidation resistance.
In general, further, there is known a technique of employing a Ti—Ni-based brazing filler metal consisting of two layers, i.e., a Ti or Ti-based alloy layer and an Ni or Ni-based alloy layer having high corrosion resistance as a brazing composite material prepared by rolling/bonding without employing the aforementioned liquid quenching method. Such a technique is disclosed in Japanese Patent Laying-Open No. 2003-117678, for example. In this Japanese Patent Laying-Open No. 2003-117678, an Ni—Cr—Fe-based anticorrosion/heat-resistant superalloy is listed as an Ni alloy.
However, the Mn—Ni—Cu alloy constituting part of the conventional brazing filler metal disclosed in the aforementioned International Patent Laying-Open No. WO00/18537 has such a problem that it is difficult to improve corrosion resistance of a portion bonded by brazing/bonding since Mn and Cu having low corrosion resistance are contained in the alloy.
Further, the conventional Ti—Ni-based brazing filler metal disclosed in the aforementioned Japanese Patent Laying-Open No. 2003-117678 has such a problem that it is difficult to obtain high oxidation resistance since no oxide film (passive film) of Cr2O3 is formed on the surface of a portion bonded by brazing/bonding, although the same has high corrosion resistance. Also when the Ni—Cr—Fe-based anticorrosion/heat-resistant superalloy listed in Japanese Patent Laying-Open No. 2003-117678 as the Ni alloy is employed, the rate of reaction between the brazing filler metal and the Ni—Cr—Fe-based anticorrosion/heat-resistant superalloy in brazing is retarded since the melting temperature of the Ni—Cr—Fe-based anticorrosion/heat-resistant superalloy is high (about 1800° C. to about 2000° C.). Thus, the rate of diffusion of Cr from the Ni—Cr—Fe-based anticorrosion/heat-resistant superalloy into the brazed/bonded portion is retarded, and hence there is such inconvenience that Cr is not sufficiently supplied to the brazed/bonded portion. Thus, there is such a problem that it is difficult to obtain high oxidation resistance since no oxide film (passive film) of Cr2O3 of a sufficient quantity is formed on the surface of the portion bonded by brazing/bonding.